leetcode:112 Path Sum－每日編程第二十七題

Path Sum

Total Accepted: 81534 Total Submissions: 268857 Difficulty: Easy

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

For example:
Given the below binary tree and sum = 22,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \      \
        7    2      1

return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

思路：

注意val可能爲正，可能爲負。遞歸即可解決。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool hasPathSum(TreeNode* root, int sum,const int& expected){
        if(root==NULL){
            return false;
        }
        sum+=root->val;
        if(sum==expected){
            if(root->left==NULL&&root->right==NULL){
                return true;
            }
        }
        return hasPathSum(root->left,sum,expected)||hasPathSum(root->right,sum,expected);
    }

    bool hasPathSum(TreeNode* root, int sum) {
        if(root==NULL){
            return false;
        }
        return hasPathSum(root, 0, sum);
    }
};